The present invention relates to a surface-emitting laser (SEL) and a fabrication method thereof, and specifically to a vertical-cavity surface-emitting laser (VCSEL) which is fabricated by using a selective oxidation process, and a fabrication method thereof.
According to a VCSEL, not only a light beam having a circular cross section can be obtained, but also a plurality of light emitting parts can be two-dimensionally integrated in high density on a single substrate. In addition, the VCSEL can be operated with low power consumption and fabricated at low cost. For such features, the VCSEL has been drawing attention as an optical source for optical communication and optical information processing in the next generation, and many investigations and development have been made for it so far.
Recently, researches have been actively conducted to improve VCSEL performance by selectively oxidizing semiconductor layers, such as AlGaAs layers, which included in a GaAs-based VCSEL mirror. One example of a VCSEL fabricated with this selective oxidation process is disclosed in Electronics Letters, 31 (1995). p. 560-562.
FIG. 9 is a schematic cross section showing the construction of the VCSEL disclosed in the above-mentioned literature. A VCSEL 900 shown in FIG. 9 comprises a vertical cavity which is disposed on an n-type GaAs substrate 910 and which includes n-DBR 920, InGaAs/GaAs strained quantum well 930, and p-DBR 940 which are stacked in this order on the substrate 910. Each of the p-type DBR and the n-type DBR has a multi-layer structure where GaAs layers 941 and AlAs/Al.sub.x O.sub.y layers 942 are deposited alternately, and this multi-layer structure is etched in the form of a mesa down to the substrate 910. Each of the AlAs/Al.sub.x O.sub.y layers 942 is composed of an AlAs region which is located in the mesa center, and an Al.sub.x O.sub.y region which surrounds the AlAs region. The Al.sub.x O.sub.y region is formed after the mesa etching process by oxidizing the AlAs layers in parts laterally from the mesa side wall. The entire mesa is protected by a polyimide 950. The output light 960 is outputted from the bottom surface of the substrate 910.
The following is a description of the operation of the VCSEL 900.
Since the Al.sub.x O.sub.y regions (the shaded portions in FIG. 9) are insulators, a current flows through the narrow AlAs regions in the mesa center (white portions). As a result, electrical confinement effect is enhanced. In the AlAs/Al.sub.x O.sub.y layers 942, the AlAs regions have different refractive index from the Al.sub.x O.sub.y regions, so that an index guided waveguide structure is formed and lateral optical confinement is realized. Due to the electrical and optical confinement effect, a substantial decrease in the threshold current is expected.
On the mesa side wall, a thin damage layer is formed by mesa etching, which contains non-radiative recombination centers. For this reason, the current which flows in the vicinity of the mesa side wall becomes non-radiative current which does not contribute to light emission. Since this structure makes the current flows through the mesa center only, so as to make the non-radlative current nearly zero, a further decrease in the threshold current can be expected. The VCSEL 900 has realized the threshold current 70 .mu.A which is the world-smallest value.
Another conventional example is disclosed in Applied Physics Letter, 66 (1995), p.3413-3415. FIG. 10 is a schematic cross section showing the construction of a VCSEL 1000 which is disclosed in the prior art document.
The VCSEL 1000 comprises an active layer 1020 and a p-DBR 1030 which are stacked in this order on an n-DBR 1010, and forms a mesa by etching down to the n-DBR 1010. A ring-shaped p-side electrode 1040 is formed on the p-DBR 1030. The p-DBR 1030 has a multi-layer structure which is composed of AlGaAs layers and GaAs layers deposited alternately. Only the AlGaAs layer which is lowermost in the multi-layer structure is composed of an Al.sub.0.98 Ga.sub.0.02 As layer 1032 where Al mole fraction=0.98. The other layers are each composed of an AlGaAs layer and a GaAs layer where Al mole fraction=0.9. As a result, an Al.sub.0.9 Ga.sub.0.1 As/GaAs DBR 1033 functions as a top mirror.
In this prior art, only the Al.sub.0.98 Ga.sub.0.02 As layer 1032 is selectively oxidized by using the oxidation rate difference between Al.sub.0.98 Ga.sub.0.02 As and Al.sub.0.9 Ga.sub.0.1 As (about 15:1), so as to form the Al.sub.x O.sub.y region 1031.
The VCSEL 1000 operates in the same manner as the VCSEL 900 shown in FIG. 9, and offers a lower threshold current.
According to the above-mentioned first prior art, the obtained optical output has only several .mu.W order. The second prior art employs a hybrid mirror structure which uses two types of AlGaAs layers having different Al mole fractions from each other in order to oxidize only one layer. To be more specific, an AlGaAs layer (having a larger Al mole fraction) which is susceptible to oxidation is used as a layer to be selectively oxidized whereas the other AlGaAs layers which are not oxidized have a smaller Al mole fraction. According to this construction, it is necessary to use AlGaAs layers (having a smaller Al mole fraction) whose refractive index is closer to that of the GaAs layers than that of AlGaAs layers having a large Al mole fraction for the most part of the mirror. In order to obtain a sufficient reflectivity when the refractive index difference between the GaAs layers and AlGaAs layers which compose a mirror is small, a mirror must include more layers than in the case where AlGaAs layers with a large Al mole fraction are used. This not only raises the manufacturing cost but also increases the mirror resistance.